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The mark on one side of your lover's face caused by long vigorous heterosexual sex in the missionary position. Characteristically shown by a red beard-burn on the apple of the cheek and black eye make-up that has smudged around the respective side, giving a bruising effect. The "Mark" can often times be confused as spousal abuse as it looks like the bitch just got punched in the face.

"I'm worried about your wife...do you beat her?"
"No man, thats just the Mark of the Beast."
"O"

The Urban Dictionary T-Shirt

This only applies when a groupie receives a facial from three, 6 stringed musician. Preferably of the metal genre, and not necessarily all at the same time. As groupies tend to go through members, keep track of who's face you blew your load on, and compare with the other band mates. If a groupie appears on 3 of the lists, she's been marked by three six stringed instruments..... 6-6-6 ...... thus, the mark of the beast.

The Urban Dictionary T-Shirt

The sexual connotation of 'mark of the beast' involves heterosexual intercourse during the females menstrual period. Then, after gratuitous sex, the male phallus that is currently drenched in menstrual blood, marks an 'X' shape on the female, usually the forehead. She now has the mark of the beast.

"Oh, man, I would totally mark that beast!"
"I would boink my girlfriend, but I am afraid I would have to give her the mark of the beast, at least until saturday."